Optical waveguide device

Abstract: In order not to vary a light dividing ratio, a light insertion loss and a light extinction ratio, even if an environmental temperature is varied, in a first aspect, an optical waveguide device has at least one optical waveguide substrate (3) made of ferroelectric crystals, an optical waveguide formed in the optical waveguide substrate, a first crystal plane (20) formed in the optical waveguide substrate in which charges of one polarity are generated due to pyroelectrical effects, a second crystal plane (30) made of ferroelectrical crystals in which charges of the other polarity are generated, a first conductive layer (4) formed on the first crystal plane, and a second conductive layer (14) formed on the second crystal plane, wherein the first conductive layer is conducted electrically to the first conductive layer.

Description: The present invention relates to optical waveguide devices comprising an optical waveguide substrate made of ferroelectric crystals. In the fields of optical measurement systems, optical communication systems, etc., it is known to manufacture optical waveguide devices by forming three dimensional optical waveguides in various electro-optics crystals. As for the optical waveguide devices, optical branch devices, optical modulation devices, optical polarization devices, optical switches, multiplexer, and so on are known. The optical waveguide devices have such advantages as small size, good stability, low necessary power and high speed signal transmission. The inventors investigated the use of the optical waveguide device, which is manufactured by forming the three-dimensional optical waveguide of Y-branch type in the substrate made of LiNbO 3 , as a modulator for fiber optic gyroscopes. In this modulator, a light impinging upon one end of the optical waveguide is divided by a ratio of 50:50% at a branch portion of the optical waveguide. One of the divided light is phase-modulated by applying a predetermined voltage. Phase differences due to Sagnac effects, which are generated by a rotation of the objected optical systems, are detected by the optical waveguide device. The thus obtained phase difference is introduced into a formula showing the relation between a rotation angular rate of the optical system and a phase difference to obtain a rotation angular rate. The inventors further investigated an intensity modulator of Mach-Zehnder type formed by arranging an optical waveguide of Mach-Zehnder type in the substrate. In this intensity modulator, a light impinging upon the intensity modulator is divided. Respective divided lights are phase-modulated by applying a voltage thereto. Then, the modulated lights are interfered to vary the intensity. During these investigations, the inventors faced to the problems mentioned below. In the modulator for fiber optic gyroscope, it is necessary to set the dividing ratio substantially equal to 50:50. However, especially in the modulator for fiber optic gyroscope, an admission range of the dividing ratio is very narrow. This is because, in the case that the dividing ratio is out of 50:50, the detected rotation angular rate is largely varied even if the rotation rate of the optical system is not varied. For example, if it is assumed that the dividing ratio is temporarily varied from 50:50 to 40:60, the obtained rotation angular rate is varied about 20%. Therefore, in the modulator for fiber optic gyroscope, it is required to set the dividing ratio within 48:52∼50:50. Moreover, the optical waveguide device is principally required to act stably along a wide temperature range. However, if an environmental temperature of the optical waveguide device was varied, the dividing ratio at the branch portion of the optical waveguide was temporarily varied largely. As a result, the dividing ratio of 50:50 was varied to 30:70, especially to substantially 100:0. Moreover, in addition to the light dividing ratio variation, a light insertion loss of the optical waveguide device occurred. Further, in the Mach-Zehnder intensity modulator, if the light dividing ratio was varied, an extinction ratio is varied largely, and the light insertion loss of the overall optical waveguide device is temporarily increased at the same time. In connection with the pyroelectric effect, which is generation of charges in crystal faces in a spontaneous electrical polarization direction, due to variation of environmental temperature, JP-A-5-88125 describes that a buffer layer is formed on a surface of an optical waveguide, and an antistatic layer is formed on the buffer layer. Moreover, a control electrode is formed on the antistatic layer. However, in this construction the charges generated by the pyroelectrical effects are deviated non-uniformly in the crystal surface due to the effect of the control electrode. The antistatic layer only prevents the charge non-uniformity, and does not function to neutralize instantaneously the charges generated by the pyroelectrical effects. Further, the optical waveguide has a linear shape, and thus there is no problem about a variation of the light dividing ratio. Moreover, in JP-A-2-257108, it is described that a pair of ferroelectric substrates are connected via a conductive member in such a manner that respective spontaneous electric polarization are reversed with each other so as to reduce the pyroelectrical effects. However, in this invention, the charges generated in connection surfaces of the ferroelectric substrates can be neutralized, but the charges generated in the other surfaces than the connection surfaces cannot be neutralized. Therefore, electric fields remain in the ferroelectric substrates, and thus the pyroelectrical effects cannot be eliminated. Further, the optical waveguide has a linear shape, and thus there is no problem about a variation of the light dividing ratio. EP-A-490387 describes a waveguide substrate in which opposed Z-surfaces at which electrical charges of opposite polarity arise due to the pyroelectric effect, are connected together by a conductor on the bottom face. The waveguide is in the top face, which is an X-surface. In this way the surface charges are neutralized. JP-A-4-104221 discloses a waveguide device in which the waveguide is in a Z-surface of the substrate. To prevent the pyroelectric effect affecting performance, an overlying dummy substrate of the same material is adhered to this surface by means of electrodes at parts of the opposed surface areas. The dummy substrate has its polarization direction the same as that of the waveguide substrate, so that opposite charges are generated in these opposed surfaces. It is an object of the invention to provide an optical waveguide device comprising an optical waveguide substrate made of ferroelectric crystals, and an optical waveguide formed in the optical waveguide substrate, in which for example a light dividing ratio and a light insertion loss are not varied, even if an environmental temperature of the optical waveguide device is varied. According to the invention there is provided an optical waveguide device as set out in claim 1. Exemplary embodiments of the invention will now be described with reference to the following figures. Fig. 1 is a plan view showing one form of an optical waveguide device which is illustrated for comparison with the devices of the invention; Fig. 2 is a plan view illustrating one embodiment of the optical waveguide device according to the invention; Figs. 3a and 3b are side views respectively depicting the optical waveguide device shown in Fig. 2; Fig. 4 is a side view showing another embodiment of the optical waveguide device according to the invention; Fig. 5a is a side view illustrating another embodiment of the optical waveguide device according to the invention, and Fig. 5b is a cross sectional view cut along Vb-Vb line in Fig. 5a; Fig. 6a is a side view depicting another embodiment of the optical waveguide device according to the invention, and Fig. 6b is a cross sectional view cut along VIb-VIb line in Fig. 6a; Figs. 7a and 7b are side views respectively showing another embodiment of the optical waveguide device according to the invention; Fig. 8a is a side view illustrating another embodiment of the optical waveguide device according to the invention, and Fig. 8b is a cross sectional view cut long VIIIb-VIIIb line in Fig. 8a; Fig. 9 is a plan view depicting another embodiment of the optical waveguide device according to the invention; Fig. 10 is a plan view showing another embodiment of the optical waveguide device according to the invention; Fig. 11 is a plan view illustrating another embodiment of the optical waveguide device according to the invention; Fig. 12 is a plan view depicting an apparatus for measuring a light insertion loss and a light dividing ratio with respect to the optical waveguide device; Fig. 13 is a graph showing a relation between the environmental temperature and the light insertion loss or the light dividing ratio in the comparative example; Fig. 14 is a graph illustrating a relation between the environmental temperature and the light insertion loss or the light dividing ratio. In one form of waveguide device of the invention, a dummy substrate made of ferroelectric crystals is formed apart from the optical waveguide substrate. In this case, the following effects can be obtained. In this case, since conduction patterns are not formed on the optical waveguide, the conduction patterns do not affect the optical waveguide. Moreover, it is not necessary to increase the manufacturing steps, as compared with the case such that the conduction patterns are formed on the reverse major plane on which no optical waveguide is formed. This is because, since both of the conductive layer in the dummy substrate and the conductive layer in the optical waveguide substrate are formed to side planes perpendicular to the major plane in which the pyroelectrical effects occur, the conductive layers can be formed at the same time in the same vapor apparatus or in the same printing apparatus. In this case, it is preferred to stack the optical waveguide substrate and the dummy substrate, in such a manner that the respective crystal planes connected together are aligned in the same side in each case, and to form the conductive layers continuously. If the conductive layers are formed integrally in this manner, the forming step of the conductive layer can be performed under the same condition, and thus a large scale manufacturing can be performed. Further, if the dummy substrate and the optical waveguide substrate are integrated in this manner, the following effects can be obtained. In an actual use, the optical waveguide substrate is fixed to a package having predetermined dimensions, and the package is used as optical devices. In this case, in case that the optical waveguide substrate is fixed directly to the package by means of adhesives etc., if the environmental temperature is varied, a stress is applied to the optical waveguide substrate via the adhesive layer, and the characteristics of the optical waveguide are varied accordingly due to the applied stress. This problem can be eliminated by integrating the dummy substrate and the optical waveguide substrate and fixing the dummy substrate to the package. Moreover, the effects mentioned above can be obtained in the optical waveguide device having a plurality of optical waveguide substrates with crystal planes of the optical waveguide substrates connected to crystal planes of one dummy substrate. In this case, it is possible to decrease the number of dummy substrates. Further, it is possible to stack the dummy substrate between two optical waveguide substrates, in such a manner that respective crystal planes connected together are aligned in the same side, and to form the conductive layers continuously. In this case, it is possible to stack optical waveguide substrates and dummy substrates alternately. As mentioned above, in the case that the conductive layer is formed continuously in the optical waveguide substrates and the dummy substrates, it is preferred to arrange the adhesive layers between the optical waveguide substrates and the dummy substrates. In this case, since the adhesive force between the optical waveguide substrate and the dummy substrate can be made larger, the conductive layers can be formed easily. Especially, in the case that the conductive layer is formed by the vapor deposition method, since the conductive layer does not function to fix the optical waveguide substrate and the dummy substrate with each other, it is necessary to arrange the adhesive layer between the optical waveguide substrate and the dummy substrate. Moreover, in the case that the integrated conductive layer is formed by arranging a conductive paste on the optical waveguide substrate and the dummy substrate and firing the conductive paste, if a thickness of the conductive paste is made thick, the connection and integration between the optical waveguide substrate and the dummy substrate can be performed by the conductive layers. Further, in the invention, it is possible to arrange a plurality of optical waveguide substrates without using the dummy substrates mentioned above and to electrically connect the crystal planes of one optical waveguide substrate to the crystal planes of the other optical waveguide substrate. In this case, the same effects as those of the case using the dummy substrate can be obtained. Further, since it is not necessary to use the dummy substrate which does not function as the optical device, it is a preferred embodiment. Moreover, it is possible to stack a plurality of optical waveguide substrates, in such a manner that the respective crystal planes connected together are aligned in the same side in each case, and to form the conductive layers on the crystal planes continuously. The effects of the above construction has been explained. In this case, it is possible to arrange the adhesive layer between the optical waveguide substrates. Further, as mentioned above, a plurality of optical waveguide substrates can be connected by means of the fired paste. In the case that the connection and integration between the optical waveguide substrate and the dummy substrate or the connection and integration between a plurality of optical waveguide substrates is performed by the adhesive layer or the fired paste, the following effects can be obtained. That is to say, when optical fibers are connected to the optical waveguide substrate, it is necessary to connect respective optical waveguide substrates to an optical fiber support substrate as mentioned below. However, since the thickness of the optical waveguide substrate is about 1 mm, a handling during the assembling operation is very difficult, and it is necessary to perform the assembling operation most carefully. In this case, if the connection and integration between the optical waveguide substrate and the other optical waveguide substrate or the dummy substrate is performed as mentioned above, the overall thickness become more than twice as compared with the thickness of the optical waveguide substrate, and thus the handling becomes easier. As for ferroelectric crystals, it is preferred to use one or more ferroelectric crystals selected from the group consisting of LiNbO 3 , LiTaO 3 , Li(Nb x , Ta 1-x ), BaTiO 3 , ZnO, NH 4 H 2 PO 4 , KH 2 PO 4 and KTiOPO 4 . Moreover, it is more preferred to use one or more ferroelectric crystal selected from LiNbO 3 , LiTaO 3 and Li(Nb x , Ta 1-x ). As for the optical waveguide, it is preferred to use (1) optical waveguide formed by thermally diffusing titanium or copper into ferroelectric crystal, or (2) optical waveguide formed by proton-exchange process in which lithium atom in LiNbO 3 crystal, LiTaO 3 crystal or Li(Nb x , Ta 1-x ) O 3 crystal is exchanged by hydrogen atom. In addition, the present invention can be applied to optical waveguides of ridge type, optical waveguides of dielectric load type, optical waveguide of metal load type, etc. In the optical waveguide device according to the invention, the optical waveguide is divided into a plurality of rows at an output side of the optical waveguide substrate. A shape of the optical waveguide may be planar or may be curvature. A branch portion of the optical waveguide may be so-called Y-branch. In this case, one row of the optical waveguide in an input side is coupled with two rows of optical waveguides in an output side, at the branch portion of these optical waveguides. Moreover, the branch portion of the optical waveguide may be formed as a so-called directional coupler. The directional coupler is formed by arranging adjacent optical waveguides very closely and functions to shift a guided light from one optical waveguide to the other optical waveguide. Therefore, a guided light can be divided by the directional coupler. Moreover, the directional coupler can be used together with the Y-branch. The optical waveguide devices according to the invention can be used for active devices and passive devices. As for the passive devices, use is made of beam splitters, star couplers having a waveguide of cascade type consisting of a plurality of Y-branch portions, and so on. As for the active devices, use is made of phase modulators, optical switching device, acoustic-optics devices, logic devices for optical computers (AND element, OR element), multiple modulators having such a construction that optical modulation electrodes are arranged to the star coupler, and so on. Especially in the optical modulators for the fiber optic gyroscope, if the light dividing ratio of the divided light is out of the predetermined value, the rotation rate of the optical system to be measured is extraordinarily varied, and thus it is not possible to measure the precise values. Therefore, the optical waveguide devices according to the invention can be preferably used for the above-mentioned aims. Moreover, also in the optical waveguide devices for dividing lights such as the beam splitters, the optical switching devices, the logic devices, and so on, they output erroneous values if the divided lights are varied. Therefore, the invention can be preferably applied for such devices. Especially in the optical switching devices and the logic devices, ON-OFF states are detected from the divided lights transmitted through two rows of optical waveguides. Therefore, if the divided lights are varied over the threshold level, the ON state and the OFF state are exchanged, and thus it is not possible to use them. Further, in the star couplers and the multiple modulators, a plurality of Y-branch portions are arranged to make the cascade construction. Therefore, if the light dividing ratio of respective Y-branch portions is varied out of the predetermined value, these variations are multiplied from input to output. Therefore, even if the variation of the light dividing ratio at one Y-branch portion is for example 5%, the multiplied variations become extraordinarily large value. The invention can be preferably applied to the star couplers and to multiple modulators. As for the formation of the conductive layers in the devices of the invention, use is made of the following. (1) By means of the vapor deposition method, a thin layer made of semiconductors or metals such as indium oxide, indium oxide-tin oxide, copper oxide, Si/Ge, and so on is formed. (2) On the crystal plane, a paste layer including semiconductors or metals such as indium oxide, indium oxide-tin oxide, copper oxide, Si/Ge, and so on is arranged, and the arranged paste layer is fired. (3) By performing a plasma treatment or an oxidizing treatment with respect to the crystal plane, a crystal structure of the crystal plane is deteriorated to form the conductive layer. (4) A thin layer made of semiconductors or metals having conductive properties and light shielding properties such as carbon black, graphite, and so on is formed. The thin layer is formed by arranging a paste layer including carbon black or carbon on the crystal plane and firing the paste layer. Where two conductive layers are connected electrically, if a resistivity of the conducted portion is large, it is not possible to neutralize the charges generated in the first crystal plane quickly. Therefore, a resistivity R of the conducted portion should be set in such a manner that electric fields applied to the optical waveguide portion is always below 10 V/cm under thinkable temperature variation rate. Under various temperature condition, a voltage V applied to the optical waveguide portion due to the pyroelectrical effects is obtained from the following formula. V = QR/d where, Q is the charges (coulomb) generated during a unit time, which depends on the environmental temperatures, R is a resistivity (Ω) of the conducted portion and d is a distance between the first crystal plane and the second crystal plane. Accordingly, in order to limit the voltage V applied to the optical waveguide portion under thinkable temperature variation rate below 10 V/cm, 10>QR/d should be satisfied. That is to say, the resistivity R of the conducted portion should be designed to satisfy R<10d/Q. Moreover, if it is necessary that the optical waveguide device performs always in a good condition under thinkable environmental temperature, the resistivities of both of the first conductive layer and the second conductive layer may be set below 100 Ω/cm 2 . Hereinafter, embodiments of the invention will be explained with reference to Figs. 2 to 14. Fig. 1 is a plan view showing an optical waveguide device which is illustrated for comparison with the devices of the invention. In Fig. 1, an optical waveguide substrate 3 is formed by an X-cut plate made of LiNbO 3 , LiTaO 3 , Li(Nb x , Ta 1-x )O 3 , and so on. A three dimensional optical waveguide 7 is formed on one major plane 3a of the optical waveguide substrate 3. The optical waveguide substrate 3 comprises a linear portion 7a in a light input side, a Y-branch portion 7b, and a pair of linear portions 7c, 7d in a light output side. Electrodes 6A and 6C are formed parallelly in such a manner that the linear portion 7c is arranged between the electrodes 6A and 6C. Electrodes 6B and 6C are also formed parallelly in such a manner that the linear portion 7d is arranged between the electrodes 6B and 6C. Therefore, it is possible to apply a control voltage to the linear portions 7c, 7d and to modulate a phase of the light transmitted through respective linear portions 7c, 7d. An optical fiber support substrate 2A is connected to the light input side of the optical waveguide substrate 3 to support an optical fiber 1A. The optical fiber 1A and the linear portion 7a are optically coupled. Moreover, an optical fiber support substrate 2B is connected to the light output side of the optical waveguide substrate 3 to support a pair of optical fibers 1B. Respective optical fibers 1B and respective linear portions 7c, 7d are optically coupled. In the optical waveguide device shown in Fig. 1, the optical waveguide substrate 3 has a first crystal plane 20 (for example -Z plane) and a second crystal plane 30 (for example +Z plane). A first conductive layer 4 is formed on the first crystal plane 20. A second conductive layer 14 is formed on the second crystal plane 30. Negative charges are generated in the first crystal plane 20, and positive charges are generated in the second crystal plane 30. In this embodiments, the first conductive layer 4 and the second conductive layer 14 are electrically conducted via a conductive layer (conduction pattern) 5 formed on the major plane 3a. Fig. 2 is a plan view showing an embodiment of the optical waveguide device according to the first aspect of the invention. Fig. 3a is a side view viewed from a crystal plane 20B side of the optical waveguide device shown in Fig. 2, and Fig. 3b is a side view viewed from a crystal plane 20A side of the optical waveguide device shown in Fig. 2. In the embodiment shown in Figs. 2, 3a and 3b, the portions similar to those of Fig. 1 are denoted by the same numerals, and the explanations thereof are omitted here. In the optical waveguide device according to this embodiment, a dummy substrate 13 made of ferroelectric crystal is arranged under the other major plane 3b of the optical waveguide substrate 3. One major plane 13a is not opposed to the optical waveguide substrate 3, and the other major plane 13b is opposed to the major plane 3b of the optical waveguide substrate 3. Therefore, as shown in Fig. 3a, a second crystal plane 30A (-Z plane) of the dummy substrate 13 is arranged in the same side as the first crystal plane 20B (+Z plane). Moreover, as shown in Fig. 3b, a second crystal plane 30B (+Z plane) of the dummy substrate 13 is arranged in the same side as the first crystal plane 20A (-Z plane). Then, as shown in Fig. 3a, a first conductive layer 4B is formed on the first crystal plane 20B, and a second conductive layer 14A is formed on the second crystal plane 30A. Further, the first conductive layer 4B and the second conductive layer 14A are electrically conducted by conduction members 8. In Fig. 3, the conductive layers are shown by a hatched area. Also as shown in Fig. 3b, a first conductive layer 4A is formed on the first crystal plane 20A, and a second conductive layer 14B is formed on the first crystal plane 4A. Further, the first conductive layer 4A and the second conductive layer 14B are electrically conducted by the conduction members 8. In order to eliminate the pyroelectrical effects, materials and dimensions of the dummy substrate 13 should be equal to those of the optical waveguide substrate 3. However, even in the case that the material of the dummy substrate 13 is not equal to those of the optical waveguide substrate 3, it is possible to neutralize the charges generated therein by suitably adjusting the dimensions of the dummy substrate 3. Fig. 4 is a side view showing another embodiment of the invention, in which respective first crystal planes 20B of two optical waveguide substrates 3 are electrically conducted to the second crystal plane 30a of one dummy substrate 13. In Fig. 4, the portions similar to those of Fig. 1 are denoted by the same numerals, and the explanations thereof are omitted here. In this embodiment, the dummy substrate 13 is arranged between two optical waveguide substrates 3. One major plane 13a of the dummy substrate 13 is opposed to the major plane 3a of the lower optical waveguide substrate 3, and the other major plane 13b of the dummy substrate 13 is opposed to the major plane 3b of the upper optical waveguide substrate 3. Therefore, a second crystal plane 30A (-Z plane) of the dummy substrate 13 is arranged in the same sides of the first crystal planes 20B (+Z plane) of respective optical waveguide substrates 3. Moreover, a second crystal plane 30B (+Z plane) not shown of the dummy substrate 13 is arranged in the same side of the first crystal planes 20A (-Z plane) of respective waveguide substrates 3. As shown in Fig. 4, the first conductive layers 4B are formed respectively on the first crystal planes 20B, and the second conductive layer 14A is formed on the second crystal plane 30A. Further, respective conductive layers 4B and the conductive layer 14A are electrically conducted by the conduction members 8. Moreover, not shown in Fig. 4, the first conductive layers 4A are formed on respective first crystal planes 20A, and the second conductive layer 14B is formed on the second crystal plane 30B. Further,the first conductive layers 4A and the second conductive layer 14B are electrically conducted by the conduction members 8. Fig. 5a is a side view showing another embodiment of the optical waveguide device according to the invention, and Fig. 5b is a cross sectional view cut along Vb-Vb line in Fig. 5a. In this embodiment, the dummy substrate 13 is arranged under the major plane 3b of the optical waveguide substrate 3. The major plane 13b of the dummy substrate 13 is opposed to the major plane 3b of the optical waveguide substrate 3. Therefore, as shown in Fig. 5a, the second crystal plane 30A (-Z plane) of the dummy substrate 13 is arranged in the same side of the first crystal plane 20B (-Z plane). Moreover the second crystal plane 30B (+Z plane) of the dummy substrate 13 is arranged in the same side of the first crystal plane 20A (-Z plane). Further, in this embodiment, the optical waveguide substrate 3 and the dummy substrate 13 are stacked through an adhesive layer 12. The first crystal plane 20B and the second crystal 30A are arranged in the same side, and a conductive layer 11B is formed continuously on the first crystal plane 20B and the second crystal plane 30A. The first crystal plane 20a and the second crystal plane 30B are arranged in the same side, and a conductive layer 11A is formed continuously on the first crystal plane 20a and the second crystal plane 30B. Fig. 6a is a side view showing another embodiment of the optical waveguide device according to the invention, and Fig. 6b is a cross sectional view cut along VIb-VIb line in Fig. 6a. In this embodiment, the dummy substrate 13 is arranged between two optical waveguide substrates 3 through adhesive layers 12A and 12B. The second crystal plane 30A (-Z plane) of the dummy substrate 13 is arranged in the same side of respective first crystal planes 20B (+Z plane) of respective optical waveguide substrates 3. Moreover, the second crystal plane 30B (+Z plane) of the dummy substrate 13 is arranged in the same side of the first crystal planes 20A (-Z plane). Two first crystal planes 20B and the second crystal plane 30A are arranged in the same side, and a conductive layer 15B is arranged continuously on two first crystal planes 20B and one second crystal plane 30A. Moreover, two first crystal planes 20A and the second crystal plane 30B are arranged in the same side, and a conductive layer 15A is arranged continuously on two first crystal planes 20a and one second crystal plane 30B. Figs. 7a and 7b are side views respectively showing another embodiment of the optical waveguide device according to the invention. In Figs. 7a and 7b, portions similar to those of Fig. 1 are denoted by the same numerals, and the explanations thereof are omitted here. In this embodiment, no dummy substrate is used. That is to say, as shown in Figs. 7a and 7b, one optical waveguide substrate 3A and the other optical waveguide substrate 3B are stacked with each other. The major plane 3b of the optical waveguide substrate 3A is opposed to the major plane 3b of the optical waveguide substrate 3B. Therefore, as shown in Fig. 7a, the first crystal plane 20B (+Z plane) of the optical waveguide substrate 3A and the second crystal plane 30A (-Z plane) of the optical waveguide substrate 3B are arranged in the same side. In the reverse side not shown in Fig. 7a, the first crystal plane 20a (-Z plane) of one optical waveguide substrate 3A and the second crystal plane 30B (+Z plane) of the optical waveguide substrate 3B are arranged in the same side. Then, as shown in Fig. 7a, the first conductive layer 4B is formed on the first crystal plane 20B, and the second conductive layer 14A is formed on the second crystal plane 30A. Further, the first conductive layer 4B and the second conductive layer 14A are electrically conducted by the conduction members 8. Moreover, as shown in Fig. 7b, the first conductive layer 4A is formed on the first crystal plane 20A, and the second conductive layer 14B is formed on the second crystal plane 30B. Further, the first conductive layer 4A and the second conductive layer 14B are electrically conducted by the conduction members 8. Fig. 8a is a side view showing another embodiment of the optical waveguide device according to the invention, and Fig. 8b is a cross sectional view cut along VIIIb-VIIIb line in Fig. 7a. In Fig. 8a, one optical waveguide substrate 3A and the other optical waveguide substrate are arranged up and down. The major plane 3b of the optical waveguide substrate 3A is opposed to the major plane 3b of the optical waveguide substrate 3B. Therefore, the first crystal plane 20B (+Z plane) of the optical waveguide substrate 3A and the second crystal plane 30A (-Z plane) are arranged in the same side. In the reverse side not shown in Fig. 8a, the first crystal plane 20A (-Z plane) of the optical waveguide substrate 3A and the second crystal plane 30B (+Z plane) of the optical waveguide substrate 3B are arranged in the same side. Two optical waveguide substrates 3A and 3B are stacked through the adhesive layer 12 so as to integrate the optical waveguide substrates 3A and 3B. The first crystal plane 20B and the second crystal plane 30A are aligned in the same side, and he conductive layer llB is formed continuously on the first crystal plane 20B and the second crystal plane 30A. Moreover, the first crystal plane 20A and the second crystal plane 30B are aligned in the same side, and the conductive layer 11A is formed continuously on the first crystal plane 20A and the second crystal plane 30B. Figs. 9, 10 and 11 are plan views respectively showing forms of optical waveguide devices according to which the invention can be applied. In Figs. 9 and 10, use is made of a directional coupler at a light branch portion of the optical waveguide. In the embodiment shown in Fig. 9, a pair of optical waveguides 17A and 17B are formed in one major plane of the optical waveguide substrate. The optical waveguides 17A and 17B are arranged closely in a region of a couple portion 10, and a light guided through one of the optical waveguides is divided in the couple portion 10. By varying a length of the couple portion 10, it is possible to vary the light dividing ratio in a range of 0:100∼50:50. In this embodiment, lights transmitted through respective optical waveguide can be modulated by electrodes 6A, 6B and 6C. Moreover, it is possible to form the conductive layers on the side planes of the optical waveguide 3 as shown in Figs. 2 to 8. In the embodiment shown in Fig. 10, a directional coupler having three parallel waveguides is formed in the optical waveguide substrate 3. An optical waveguide 27A in a light input side is arranged near optical waveguides 27B and 27C in a light output side, in a region of a couple portion 10, and a light guided through the optical waveguide 27A is divided in the couple portion 10. In the embodiments shown in Figs. 9 and 10, if a voltage is applied to the couple portion 10, a light dividing ratio can be varied in response to the guided light to be divided. Moreover, it is possible to form the conductive layers on the side planes of the optical waveguide substrate 3 as shown in Figs. 2 to 8. In the embodiment shown in Fig. 11, the invention is applied to the star coupler mentioned above. In an optical waveguide 37, a linear portion 37a in a light input side is divided into linear portions 37c at a Y-branch portion 37b. Further, linear portions 37a are divided respectively into linear portions 37e at Y-branch portion 37d. The linear portions 37e are respectively coupled to optical fibers 1B. In this embodiment, if a voltage is applied to the linear portions 37e, it is possible to perform a multiple modulation. Moreover, it is possible to form the conductive layer on the side planes of the optical waveguide substrate 3 as shown in Figs. 2 to 8. Experiments were performed by using a measuring apparatus shown in Fig. 12. First, the optical waveguide devices according to Fig. 1 and another comparative example were accommodated in a temperature adjusting device. Properties as the phase modulator for use in fiber optics gyroscope were measured. The optical waveguide devices were manufactured in the following manner. At first, a disk-shape wafer made of LiNbO 3 having x-cut plane was prepared. The wafer had a diameter of 3 inches and a thickness of 1 mm. A titanium thin layer having a width of 3 µm and a thickness of 500 angstrom was formed on the wafer by using the photolithography technic, and a titanium component was diffused into the LiNbO 3 by effecting a heat treatment such as 1000°C×6 hours to form the titanium diffused optical waveguide 7. After that, electrodes 6A, 6B and 6C made of metal layers were respectively formed. Dimensions of respective electrodes were a length of 15 mm and a thickness of 2500 angstrom, and a gap distance between adjacent electrodes was 10 µm. From this wafer, the optical waveguide substrates 3 for one chip were cut out. Then, the conductive layers 4 and 14 were formed. As for the material of respective conductive layers, use was made of a conductive paste (silver paste), and the conductive paste layer was formed by spraying method. A resistivity rate of respective conductive layers was not more than 10 Ω/cm 2 . As for the material of the conduction layer 5, use was made of a conductive paste (silver paste), and the conductive paste layer was formed by the spraying method. A resistivity of the conduction layer 5 was not more than 1 Ω. To the both ends of the optical waveguide substrate 3, the optical fibers 1A and 1B were connected as shown in Fig. 1. At the same time, the optical waveguide devices according to another comparative example were manufactured in the same manner as mentioned above. However, the specimens of this other comparative example have no conductive layers 4, 14 and no conductive layer 5. Then, a light having a wavelength of 0.85 µm was introduced from a light source 41, and output lights transmitted through a pair of optical fibers 1B were measured by light amount measuring apparatuses 22, 23. After that, a light insertion loss and a light dividing ratio were calculated from respective measured values of respective light amount measuring apparatuses. During this measuring operation, as shown in Figs. 13 and 14, the environmental temperature of the optical waveguide devices were varied. That is to say, the environmental temperature was varied stepwise from 20°C to -40°C. In this case, a holding time at respective temperatures was about 45 minutes, and an ascending rate and a descending rate of temperature was 80°C/hour. As a result, Fig. 13 shows the results of the comparative example, and Fig. 14 shows the results of the example according to Fig. 1. As shown in Fig. 13, in the comparative example, when the environmental temperature ascended or descended, both of the light insertion loss and the light dividing ratio were varied largely. It should be noted that, in Fig. 13, a region of the light insertion loss more 4dB and a region of the light dividing ratio more than 30:70 were cut out. Contrary to this, in the example according to Fig. 1, both of the light insertion loss and the light dividing ratio were not varied as shown in Fig. 14. In this case, a variation of the light insertion loss was less than 0.4dB, and a variation of the light dividing ratio was in a range of 50:50∼52:48. Moreover, the optical waveguide devices according to Fig. 1 and according to the comparative example were assembled in the fiber optics gyroscopes as modulators. Then, as shown in Figs. 13 and 14, the environmental temperature was varied. As a result, in the comparative example, the calculated rotation angular rate was varied by more than 20%, and sometimes it was not possible to measure the rotation angular rate. In the example according to the first aspect of the invention, the calculated rotation angular rate was varied by 4%. Further, the optical waveguide devices shown in Figs. 2, 3 and 7 according to the invention were manufactured in the same manner mentioned above. Then, the same experiment was performed with respect to respective optical waveguide devices. As a result, the same results as those shown in Fig. 14 were obtained. As mentioned above, according to the invention, it is possible to obtain the optical waveguide device showing no variations of the light dividing ratio and no variation of the light insertion loss even if the environmental temperature is varied.

Claims: An optical waveguide device comprising at least one first optical waveguide substrate (3) made of ferroelectric crystals, first and second opposite crystal planes (20A,20B) in said optical waveguide substrate (3) in which charges of respectively positive and negative polarity are generated due to the pyroelectrical effect, an optical waveguide (7) formed in said waveguide substrate (3) at a crystal plane thereof perpendicular to said first and second crystal planes, and first and second conductive layers (4A,4B,11A,11B,15A,15B) formed on said first and second crystal planes (20A,20B) respectively, characterised in that the optical waveguide device further comprises at least one further substrate made of ferroelectric crystals, which substrate is a dummy substrate (13) or a further optical waveguide-containing substrate (3B), said further substrate having third and fourth crystal planes (30A) in which charges respectively of positive and negative polarity are generated due to the pyroelectrical effect, and third and fourth conductive layers (4A,48,11A,11B,14A,14B,15A,15B) formed on said third and fourth crystal planes, said third and said fourth conductive layers being electrically connected to respectively said second and said first conductive layers, whereby in each case cancellation of charges generated by the pyroelectric effect takes place. An optical waveguide device according to claim 1, wherein said at least one first optical waveguide substrate (3) and said at least one further substrate (3,13) are stacked in such a manner that said first and fourth crystal planes are aligned at the same side of the substrates, and said second and third crystal planes are aligned together at the opposite side of the substrates, and said first and fourth conductive layers and said second and third conductive layers are in each case a continuous element. An optical waveguide device according to claim 2, wherein an adhesive layer (12) is arranged between said optical waveguide substrate (3) and said further substrate. An optical waveguide device according to any one of claims 1, 2 or 3, having a plurality of said first optical waveguide substrates (3) and wherein said at least one further substrate is a dummy substrate (13). An optical waveguide device according to claim 4, wherein said dummy substrate (13) is arranged between said first optical waveguide substrates in such a manner that said first crystal planes (20B) of the first optical waveguide substrates and said fourth crystal plane of the dummy substrate are aligned at the same side of the substrates, and said second crystal planes of the first optical waveguide substrates and said third crystal plane of said dummy substrate are aligned at the opposite side of the substrate, and said first and fourth conductive layers and said second and third conductive layers are in each case a continuous element.

Inventors: ASANO MASATO, EJIRI TETSUYA, ASANO, MASATO, EJIRI, TETSUYA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1994
Main classification: G02F1
IPC codes: G02F1